Process for the production of a multilayer protective and/or decorative coating on a substrate surface, and aqueous paints suitable for carrying out the process

ABSTRACT

The invention relates to a process for the production of two-coat finishes, in which a pigmented aqueous basecoat is applied, the basecoat obtained is coated over with a clearcoat, and basecoat and clearcoat are baked together. The process according to the invention is distinguished in that the aqueous basecoat contains a polyacrylate resin which has been prepared using furfuryl acrylate and/or furfuryl methacrylate.

The invention relates to a process for the production of a multilayerprotective and/or decorative coating on a substrate surface, in which

(1) a pigmented aqueous basecoat which contains a polyacrylate resin asbinder is applied to the substrate surface, the polyacrylate resin beingobtainable by polymerization of (a) a mixture of esters of methacrylicacid and/or esters of acrylic acid, together if required with (b)further monomers different from (a) or mixtures of such monomers,

(2) a polymer film is formed from the basecoat applied in stage (1),

(3) a transparent topcoat is applied to the resulting basecoat andsubsequently

(4) basecoat and topcoat are baked together.

The invention also relates to aqueous paints suitable for carrying outthe process described above.

The above-described process for the production of multilayer protectiveand/or decorative coatings is known as the basecoat/clearcoat processand is employed in particular in the production of finishes, inparticular metallic finishes, on car bodies (cf. e.g. DE-A-38 41 540,EP-A-287 144, DE-A-36 28 124, EP-A-195 931 and EP-A-256 540).

The extent of the metallic effect in the multilayer coatings produced bythe process under discussion depends on how many of the metallic pigmentparticles present in flake form and contained in the basecoat arepresent in the baked finish in parallel orientation to the substratesurface. The orientation of the metallic pigment particles can bedisturbed in particular after the application of the transparent topcoatand/or during the baking procedure.

The object on which the present invention is based consists in theprovision of a process of the type described above with which multilayercoatings can be obtained having an improved metallic effect incomparison to multilayer coatings of the prior art. This object issurprisingly achieved in that, in the pigmented basecoat, a polyacrylateresin is employed which can be obtained by employing as component (a) amixture of esters of methacrylic acid and/or esters of acrylic acidwhich contains furfuryl acrylate and/or furfuryl methacrylate.

The polyacrylate resins employed in accordance with the invention can beprepared by polymerization of (a) a mixture of esters of methacrylicacid and/or esters of acrylic acid, together if appropriate with (b)further monomers different from (a) or mixtures of such monomers. It isessential to the invention to employ as component (a) a mixture ofesters of methacrylic acid and/or esters of acrylic acid, which containsfurfuryl acrylate and/or furfuryl methacrylate, preferably furfurylmethacrylate. It is preferred for component (a) to contain a quantity offurfuryl acrylate and/or furfuryl methacrylate such that the proportionof furfuryl acrylate and/or furfuryl methacrylate, based on the totalweight of (a)+(b)=100% by weight, is 0.25 to 10% by weight, preferably1.0 to 9.0% by weight and particularly preferably 2.0 to 7.0% by weight.The polymerization of components (a) and (b) can be carried out in anorganic solvent or in a mixture of organic solvents. The solution of thepolyacrylate resin obtained in this way can then be converted to anaqueous dispersion by adding water. The polyacrylate resins employed inaccordance with the invention can also be prepared by polymerization ofcomponents (a) and (b) in an aqueous emulsion. Both the polymerizationin organic solvents and the polymerization in aqueous emulsion are ingeneral carried out by free-radical methods. The above-describedpolymerization processes are so well known that they require no furtherdescription here.

In order to obtain water-dilutable polyacrylate resins, components (a)and/or (b) must comprise monomers containing hydrophilic groups, forexample carboxyl groups, sulfonic acid groups or hydrophilic polyetherchains, for example --(CH₂ --CH₂ --O)_(n) H, --(--CHCH₃ --CH₂ --O)_(n) Hor --(CH₂ --CH₂ --O)_(x) (CHCH₃ --CH₂ --O)_(y) H, in which n is a numberfrom 4 to 20, x is a number from 1 to 19 and y is a number from 1 to 19.Polyacrylate resins containing carboxyl groups are in generalneutralized, at least in part. As neutralizing agents it is preferred toemploy tertiary amines. If the polyacrylate resins employed inaccordance with the invention contain carboxyl groups, then they shouldhave an acid value of 1 to 80, preferably 5 to 50 mg of KOH per g ofsolid resin.

Component (a) can contain, in addition to the furfuryl acrylate and/orfurfuryl methacrylate employed in accordance with the invention, furtheresters of methacrylic acid or acrylic acid, for example aliphatic andcycloaliphatic acrylates or methacrylates having up to 20 carbon atomsin the alcohol residue, such as methyl, ethyl, propyl, butyl, hexyl,ethylhexyl, stearyl, lauryl and cyclohexyl acrylate or methacrylate andhydroxyalkyl esters of acrylic acid or methacrylic acid, such as2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropylacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate,2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutylmethacrylate etc.

Component (b) comprises monomers which are different from the monomerscontained in component (a), or mixtures of such monomers. Examples ofmonomers from which component (b) can be comprised are: acrylic acid,methacrylic acid, styrene, a-alkylstyrene, vinyltoluene, acrylamide,methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide,acrylonitrile, methacrylonitrile, ethacrylic acid and esters ofethacrylic acid, crotonic acid and esters of crotonic acid,acrylamidomethylpropanesulfonic acid, maleic acid and esters of maleicacid, fumaric acid and esters of fumaric acid, and itaconic acid andesters of itaconic acid. It is preferred not to employ acrylamide and/ormethacrylamide in component (b).

The polyacrylate resins employed in accordance with the inventionpreferably contain hydroxyl groups, and preferably have hydroxyl valuesof between 2 and 100, particularly preferably between 10 and 60.

The number-average molecular weight (determined by gel permeationchromatography using a polystyrene standard) of the polyacrylate resinsemployed in accordance with the invention is in general between 1500 and2,000,000, preferably between 200,000 and 2,000,000, particularlypreferably between 300,000 and 1,500,000.

It is preferred to employ polyacrylate resins which have been preparedby emulsion polymerization. It is very particularly preferred to employpolyacrylate resins which can be prepared by a two-stage emulsionpolymerization in which

(α) in a first stage, 10 to 90% by weight, preferably 35 to 65% byweight, of the total amount of component (a) to be employed, together ifappropriate with 0 to 100% by weight of the total amount of component(b) to be employed, are polymerized in the aqueous phase in the presenceof one or more emulsifiers and of one or more free-radical initiators,the monomers employed in the first stage being chosen such that in thefirst stage a polymer with a glass transition temperature (T_(G1)) of+30° to +110° C., preferably +60° to +95° C., is obtained, and

(β) after at least 60% by weight of the monomers employed in the firststage have reacted, in a second stage 90 to 10% by weight, preferably 65to 35% by weight, of the total amount of component (a) to be employed,together if appropriate with 0 to 100% by weight of the total amount ofcomponent (b) to be employed are polymerized in the presence of thepolymer obtained in the first stage; the monomers employed in the secondstage are chosen such that the polymerization only of the monomersemployed in the second stage would lead to a polymer having a glasstransition temperature (T_(G2)) of -60° to +20° C., preferably -50° to0° C., the reaction conditions are chosen such that the polyacrylateresin obtained has a number-average molecular weight of 200,000 to2,000,000, preferably 300,000 to 1,500,000 and the monomers employed inthe first and in the second stage are selected such that thepolyacrylate resin obtained has a hydroxyl number of 0 to 100,preferably 10 to 60 and the difference T_(G1) -T_(G2) is 10° to 170° C.

In order to prepare the polyacrylate resins which are preferablyemployed and which can be prepared by the two-stage emulsionpolymerization described above, component (a) is divided into two parts.10 to 90% by weight of the total amount of component (a) to be employedare employed in the first stage and 90 to 10% by weight of the totalamount of component (a) to be employed are employed in the second stage.If component (b) is also employed in the preparation of the polyacrylateresin, then component (b) can be employed in the first stage, in thesecond stage or both in the first and in the second stage. The furfurylacrylate and/or furfuryl methacrylate to be employed in accordance withthe invention can be employed in the first stage, in the second stage orboth in the first and in the second stage. It is preferably employed inthe second stage. In the preparation of the polyacrylate resins whichare preferably employed and which can be prepared by the emulsionpolymerization described above, the monomers employed in the first stagemust be chosen, with regard to their nature and amount, such that in thefirst stage a polymer having a glass transition temperature (T_(G1)) of+30° to +110° C., preferably +60° to +95° C., is obtained; the monomersemployed in the second stage must be chosen, with regard to their natureand amount, such that the polymerization of only the monomers employedin the second stage would lead to a polymer having a glass transitiontemperature (T_(G2)) of -60° to +20° C., preferably -50° to 0° C., andthe monomers employed in the first and second stage must be chosen, withregard to their nature and amount, such that the condition T_(G1)-T_(G2) =10° to 170° C., preferably 80° to 150° C., is met.

The selection of the monomers to be employed in the two stages caneasily be made by the person skilled in the art since the glasstransition temperature of polyacrylate resins can be calculatedapproximately by the equation ##EQU1## where

T_(G) =glass transition temperature of the copolymer in K

W_(n) =weight fraction of the nth monomer

T_(Gn) =glass transition temperature of the homopolymer of the nthmonomer

x=number of different monomers

and because the hydroxyl value of the polyacrylate resins can becontrolled by the amount of hydroxyl group-containing monomers employed.

The person skilled in the art is familiar with how to choose thereaction conditions during the emulsion polymerization so thatpolyacrylate resins which have the number-average molecular weightsindicated above are obtained (cf. e.g. Chemie, Physik und Technologieder Kunststoffe in Einzeldarstellungen, Dispersionen synthetischerHochpolymerer, Part 1 by F. Holscher, Springer Verlag, Berlin,Heidelberg, N.Y., 1969).

It is preferred in the first stage to employ no monomers containinghydroxyl and/or carboxyl groups.

The preparation of polyacrylate resins by two-stage emulsionpolymerization is described in detail in DE-A-38 41 540 on page 2, line38 to page 5, line 7 and on page 8, line 35 to page 10, line 2.

The basecoats can also contain, in addition to the polyacrylate resinsdescribed above, other binder constituents, such as water-dilutablepolyester resins, water-dilutable polyacrylate resins not prepared usingfurfuryl acrylate and/or furfuryl methacrylate, water-dilutablepolyurethane resins, water-dilutable polyethers etc. and crosslinkingagents such as amino resins, especially melamine resins and blockedpolyisocyanates. Examples of these binder constituents and crosslinkingagents are described in, for example, DE-A-38 41 540, EP-A-287 144,EP-A-38 127, DE-A-36 28 124, EP-A-195 931 and EP-A-256 540.

As pigments, the basecoats employed in accordance with the invention cancontain all inorganic or organic pigments suitable for aqueous paints,or mixtures of such inorganic and/or organic pigments. Examples ofpigments which can be employed are titanium dioxide, iron oxide, carbonblack, metallic pigments, especially aluminum pigments, and pearlescentand interference pigments. The basecoats preferably contain metallicpigments, in particular aluminum pigments on their own or in combinationwith nonmetallic pigments.

The basecoats employed in accordance with the invention may in principlebe coated over with all transparent topcoats (clearcoats) suitable forthe basecoat/clearcoat process. The transparent topcoats may beconventional finishes--that is, containing exclusively organicsolvents--or aqueous finishes, or powder coatings.

The invention is illustrated in more detail in the following examples.All data on parts and percentages are by weight, unless expressly statedotherwise.

A) Preparation of an aqueous dispersion of a polyacrylate resin inaccordance with DE-A-38 41 540 (cf. DE-A-38 41 540, page 8, emulsionPolymer dispersion 1)

1344 g of deionized water and 12 g of a 30% strength aqueous solution ofthe ammonium salt of penta(ethylene glycol) nonylphenyl ether-sulfate(Fenopon® EP 110 from GAF Corp., emulsifier 1) are placed in acylindrical jacketed glass vessel with a stirrer, reflux condenser,stirrable feed vessel, dropping funnel and thermometer and heated to 82°C. In the stirrable feed vessel an emulsion is prepared from 720 g ofdeionized water, 24 g of emulsifier 1, 10.8 g of acrylamide, 864 g ofmethyl methacrylate and 216 g of n-butyl methacrylate. 30% by weight ofthis emulsion are added to the initial charge. Then 28% by weight of asolution of 3.1 g of ammonium peroxodisulfate (APS) in 188 g ofdeionized water are added dropwise over 5 minutes. An exothermicreaction commences. The reaction temperature is maintained at between82° and 88° C. 15 minutes after the end of the addition of the ammoniumperoxodisulfate solution the remaining 70% by weight of the emulsiontogether with the remaining 72% by weight of the ammoniumperoxodisulfate solution are added over one hour, the temperature beingmaintained at 85° C. The batch is then cooled to 82° C. and, over 2hours, a mixture of 842 g of n-butyl acrylate, 108 g of hydroxypropylmethacrylate, 43 g of methyl methacrylate, 43.2 g of methacrylic acid,32.4 g of acrylamide and 5.4 g of eicosa(ethylene glycol) nonylphenylether (Antoroxs CO 850 from GAF Corp., emulsifier 2) together with 343 gof deionized water is added. After the end of the additions the reactionmixture is held at 85° C. for a further 1.5 hours. It is then cooled andthe dispersion is poured onto a fabric with a mesh size of 30 μm. Thefinely particulate dispersion obtained has a nonvolatile content of 45%by weight, a pH of 3.4, an acid number of 13 and an OH number of 20.

B) Preparation of an aqueous dispersion of a polyacrylate resin usingfurfuryl methacrylate

1344 g of deionized water and 12 g of a 30% strength aqueous solution ofthe ammonium salt of penta(ethylene glycol) nonylphenyl ether-sulfate(Fenopon® EP 110 from GAF Corp., emulsifier 1) are placed in acylindrical jacketed glass vessel with a stirrer, reflux condenser,stirrable feed vessel, dropping funnel and thermometer and heated to 82°C. In the stirrable feed vessel an emulsion is prepared from 720 g ofdeionized water, 24 g of emulsifier 1, 10.8 g of acryl amide, 864 g ofmethyl methacrylate and 216 g of n-butyl methacrylate. 30% by weight ofthis emulsion are added to the initial charge. Then 28% by weight of asolution of 3.1 g of ammonium peroxodisulfate (APS) in 188 g ofdeionized water are added dropwise over 5 minutes. An exothermicreaction commences. The reaction temperature is held at between 82° and88° C. 15 minutes after the end of the addition of the ammoniumperoxodisulfate solution the remaining 70% by weight of the emulsiontogether with the remaining 72% by weight of the ammoniumperoxodisulfate solution are added over one hour, the temperature beingmaintained at 85° C. The batch is then cooled to 82° C. and, over 2hours, a mixture of 792 g of n-butyl acrylate, 108 g of hydroxypropylmethacrylate, 43 g of methyl methacrylate, 43.2 g of methacrylic acid,32.4 g of acrylamide, 50 g of furfuryl-methacrylate and 5.4 g ofeicosa(ethylene glycol) nonylphenyl ether (Antarox® CO 850 from GAFCorp., emulsifier 2) together with 343 g of deionized water is added.After the end of the additions the reaction mixture is held at 85° C.for a further 1.5 hours. It is then cooled and the dispersion is pouredonto a fabric with a mesh size of 30 pm. The finely particulatedispersion obtained has a nonvolatile content of 45% by weight, a pH of3.4, an acid number of 13 and an OH number of 20. This polyacrylateresin differs from the polyacrylate resin described in A) only in thatin the second stage, instead of 842 g of n-butyl acrylate, a mixture of792 g of n-butyl acrylate and 50 g of furfuryl methacrylate (FMA, RohmGmbH, CAS 3454-28-2) is employed.

C) Preparation of an aqueous basecoat using the polvacrylate resin as inA)

16.4 g of butylglycol, 3.4 g of a commercially availablemelamine/formaldehyde resin (Cymel® 301), 2.9 g of polypropylene glycol(number-average molecular weight 420) and 6.6 g of an aluminum bronze inaccordance with DE-A-36 36 183 (aluminum content: 65% by weight) arestirred using a high-speed stirrer for 15 minutes (300-500 rpm). Amixture 1 is obtained.

33.5 g of the polyacrylate resin dispersion prepared according to A) aremixed with 20.3 g of deionized water and adjusted with a 5% strengthaqueous dimethylethanolamine solution to a pH of 7.7. The mixture isstirred for 10 minutes, then 17.0 g of a 3.5% strength solution of acommercially available polyacrylic acid thickener (Viscalex® HV/30 fromAllied Colloids, pH: 8.0) are added, and the mixture is stirred for afurther 10 minutes. The mixture 2 is obtained.

Mixtures 1 and 2 are mixed for 30 minutes at 800-1000 rpm. The resultingbasecoat is adjusted with a 5% strength aqueous dimethylethanolaminesolution to a pH of 7.7 and with deionized water to a solids content of20% by weight.

D) Preparation of a basecoat according to the invention

The procedure of C) is followed. Instead of the polyacrylate resindispersion prepared according to A), the polyacrylate dispersionprepared according to B) is employed.

E) Preparation of multilayer finishes The basecoats prepared accordingto C) and D) are sprayed by a known method onto phosphatized steelpanels (Bonder 132) coated with a commercially availableelectrodeposition coating and a commercially available filler, coatedover after a flashoff time of 10 minutes at 80° C. with a commerciallyavailable clearcoat and baked for 20 minutes at 140° C. A referencevalue of 65 is determined for the metallic effect of the multilayerfinish produced with the basecoat prepared according to C) and, for themetallic effect of the multilayer finish produced with the basecoatprepared according to D), a reference value of 78. Using the basecoataccording to the invention it is therefore possible to producemultilayer finishes having a metallic effect which is improved comparedto the prior art.

The reference values for the metallic effect are determined as follows:

1. Measurement of the spectral reflection using a Datacolor/Zeiss MCS211/MMK 111 goniospectrophotometer under the following geometricalconditions

    ______________________________________                                        Geometry class Illumination                                                                            Observation                                          ______________________________________                                        25°     45°                                                                              -20°                                          70°     45°                                                                              25°                                           ______________________________________                                    

2. Determination of the CIEL*a*b* 1976 color coordinates according toDIN 5033 and DIN 6174 for a 100 standard observer under standardlighting D₆₅.

3. Determination of the reference value for the metallic effectaccording to the relationship:

reference value=((L₂₅° -L₇₀°) 50) : L₇₀°.

We claim:
 1. Process for the preparation of a multilayer coating on asubstrate surface, comprising(1) applying a pigmented aqueous basecoatwhich comprises a polyacrylate resin and a crosslinking agent selectedfrom the group consisting of aminoplasts and blocked polyisocyanates, asbinder to the substrate surface, wherein the polyacrylate resin isobtained by polymerization of(a) a mixture of esters comprisingi) estersselected from the group consisting of furfuryl acrylate, furfurylmethacrylate, and mixtures thereof and ii) esters selected from thegroup consisting of esters of methacrylic acid, esters of acrylic acid,and mixtures thereof, and (b) monomers different from a) and mixturesthereof, such that the proportion of furfuryl acrylate and/or furfurylmethacrylate, based on the total weight of (a)+(b)=100% by weight, is0.25 to 10% by weight, (2) forming a polymer film from the basecoatapplied in stage (1), (3) applying a transparent topcoat to theresulting basecoat and subsequently (4) baking together the basecoat andtopcoat.
 2. Process according to claim 1, characterized in thatcomponent (a) contains furfuryl methacrylate.
 3. Process according toclaim 1, characterized in that the pigmented aqueous basecoat contains apearlescent pigment.
 4. Aqueous paints comprising a polyacrylate resinand a crosslinking agent selected from the group consisting ofaminoplasts and blocked polyisocyanates as binder, wherein thepolyacrylate resin is obtained by polymerization of(a) a mixture ofesters comprisingi) esters selected from the group consisting offurfuryl acrylate, furfuryl methacrylate and mixtures thereof, and ii)esters selected from the group consisting of esters of methacrylic acid,esters of acrylic acid and mixtures thereof, and b) monomers differentfrom (a), such that the proportion of furfuryl acrylate and/or furfurylmethacrylate, based on the total weight of (a)+(b)=100% by weight, is0.25 to 10% by weight, characterized in that they contain a pigmentselected from the group consisting of metallic pigment pearlescentpigment, and mixtures thereof.
 5. Aqueous paints according to claim 4,characterized in that component (a) contains furfuryl methacrylate. 6.Process according to claim 1, characterized in that the pigmentedaqueous basecoat contains an aluminum pigment.
 7. Process for improvingthe metallic effect of a multilayer coating on a substrate surface,comprising(1) applying a pigmented aqueous basecoat which comprises apolyacrylate resin, a crosslinking agent selected from the groupconsisting of aminoplasts and blocked polylsocyanates, and metallicpigment, as binder to the substrate surface, wherein the polyacrylateresin is obtained by polymerization of (a) a mixture of esterscomprisingi) esters selected from the group consisting of furfurylacrylate, furfuryl methacrylate, and mixtures thereof and ii) estersselected from the group consisting of esters of methacrylic acid, estersof acrylic acid, and mixtures thereof,optionally, (b) monomers differentfrom a) and mixtures thereof, such that the proportion of furfurylacrylate and/or furfuryl methacrylate, based on the total weight of(a)+(b)=100% by weight, is 0.25 to 10% by weight, (2) forming a polymerfilm from the basecoat applied in stage (1), (3) applying a transparenttopcoat to the resulting basecoat and subsequently (4) baking togetherthe basecoat and topcoat.